Astronomers in Europe have agreed to join forces in a single project to design and build the largest optical telescope in the world.

We may be able to see Earth-like planets

Dr Tim Hawarden

Its main mirror will be up to 100 metres (330 feet) across, and will mean scientists can examine remote objects in space in much more detail.

The biggest telescopes currently in operation are between eight and 11 metres across.

Building an Extremely Large Telescope (ELT) of the size now envisioned would be a huge engineering endeavour - not least because the instrument will have to be built thousands metres above sea-level to see the heavens clearly.

Big problems

The chairman of the organising committee for the project, Professor Gerry Gilmore of Cambridge University, recognised the technical challenge involved.

Europe's existing VLT facility is high up in the Atacama desert

"The key thing about these giant telescopes is they're big - they're as big as a football pitch," he told the BBC.

"They're sitting outside, on a mountain top, where it's cold and it's windy and there are earthquakes, and it's shaking around; and it weighs up to 10,000 tonnes. So, it's sagging under its own weight.

"In spite of that, we need it to work to sufficient precision so that light is exactly in focus," he said.

One and many

The biggest optical telescope currently in operation is the 11-metre Hobby-Eberly on Mount Fowlkes in Texas, US.

The Hobby-Eberly's mirror is 11 metres across

Although the way it works means the Keck twins in Hawaii, which are 10 metres across, have a slightly larger light-collecting area.

The twins can be made to work in tandem - as a so-called interferometer - and mimic a larger telescope that is 85 metres wide.

These US 'scopes are rivalled in power by Europe's own Very Large Telescope, sited in Chile, which includes four large mirror units (each 8.2 m across) that can also be made to work as one.

But although the interferometers can match the resolution of the larger telescopes they impersonate, only the true super instruments will see the detail in the really faint objects.

Optical tricks

It would be utterly impractical, however, to build a perfect, single mirror 100 metres across and get it up a mountain.

The Kecks' primary mirrors are composed of 36 hexagonal segments

To get to the size of telescope desired, the main mirror will therefore have to be segmented - just as the mirrors in the Keck twins are.

These individual glass "tiles" - and there would be thousands of them - would be manipulated by a computer-controlled system of sensors and precision pistons to focus light on to a single point.

Optical tricks that enable current telescopes to see through the Earth's turbulent atmosphere would also be employed to enhance the image still further.

Distant planets

In Europe, several ELT projects have been under study for some years.

From this month, the two main initiatives - Euro-50, led from Sweden, and Owl, led from the European Southern Observatory (Eso) - will come together to develop a proposal for substantial additional funding from the European Union.

"We have to prove that the key technologies are viable and affordable," Dr Gilmore said.

"In particular, we have to demonstrate that the huge number of components needed for an ELT can be built taking advantage of industrial-scale efficiencies.

"The challenge is as much managerial and industrial as it is technical. But it must be met if Europe's astronomers are to have the tools they need to keep abreast of international scientific developments."

Dr Tim Hawarden, a project scientist for ELTs at the UK Astronomy Technology Centre (UK ATC) in Edinburgh, said the benefits to come from the next generation of instruments would be huge.

"For example, we may be able to see Earth-like planets, if there are any, in orbit around stars up to tens of light-years away, and perhaps even find out what their atmospheres are made of."